A synchronous motor may be defined as either a motor which has a running speed which is exactly proportional to an AC supply frequency or as a motor which is driven by waves of current synchronous with the motor back EMF. This latter definition includes at least some AC supplied brushless DC motors locked by electronic or other control to the AC supply for efficiency reasons.
AC driven synchronous motors are an efficient and well known method of powering machines requiring a constant speed drive, since the motor is locked to a rotational rate which is dependent on the driving frequency. Starting such motors is difficult since there is no unidirectional starting torque unless the motor speed is close to the synchronous speed in other words unless synchronous speed is low.
Typically this indicates the use of either a variable frequency power supply to allow starting the motor at low speed, the combination of the motor with another type of motor to supply the starting torque (such as an induction motor using at least partially the same windings), or the use of some mechanical device to provide initial movement and then to allow movement in only a single direction. Examples of the latter include various clutches and impulse starters although a feature common to many is a difficulty in determining a starting direction for the synchronous motor, which will lock equally well in either direction of rotation.
Some solutions are shown in U.S. Pat. No. 5,859,513 which uses a separate starting winding and U.S. Pat. No. 4,716,325 which uses a mechanical reverse direction protection. U.S. Pat. No. 3,529,221 shows a shaded pole motor with windings switched to start the motor, providing limited control of starting direction and torque. The performance of these motors is satisfactory only for a narrow range of undemanding applications, and construction is complex.
US patent application 2006/0071577 relates to a multipole motor in which there is a set of annular coils on either side of an annular rotor and the sets of coils are offset and separately driven so that the number of poles is effectively doubled. EP 1211787 relates to an AC motor driven by a switched DC supply derived from an AC supply such that the DC switching frequency is varied to maintain the motor at synchrony from start up speed to parity with the AC supply. EP 0991178 relates to a synchronous motor having two coils which are supplied alternately with a half cycle of one polarity of the AC supply, thereby providing a half normal synchronous speed. U.S. Pat. No. 3,529,221 relates to an AC motor having a tapped coil, part of which may act as a starting winding and the whole of which acts as the running winding. The coil parts are not separately energised at any time. U.S. Pat. No. 6,249,103 discloses a motor coil of two parts where one part is adapted to provide a higher torque than the other by using more current in dependence on the starting torque required. U.S. Pat. No. 4,695,782 relates to a motor whose rotor position is sensed by attached sensors which override the motor control if the motor moves in the incorrect direction. EP 0945973 relates to a triac driven motor with gated triac control of the start and a preferential starting position. DE 3609793 relates to a mechanical system detecting reverse rotation. JP 62171449 discloses a disk motor with electronic commutation and asymmetric windings and magnets to reduce cogging. U.S. Pat. No. 6,534,946 which relates to a motor in which reverse or forward direction is detected by an attached potentiometer or similar, but in which there is no control of torque. U.S. Pat. No. 6,249,103 relates to a motor with a tapped coil and a circuit which allows only part of the coil to operate for an initial period while starting before the whole coil is switched into circuit.
Other methods of providing motors which lock to the supply frequency involve the use of brushless DC motors which are typically multi-phase inverter driven machines with Hall effect sensors to detect the rotation of the machine rotor and a controller to bring the motor up to speed and lock the rotation to the mains supply. Such motors are costly to drive and control.
Therefore a need exists for a solution to the problem of providing a synchronous motor which will start without undue mechanical or electrical complexity and which will reach and maintain a rotational speed synchronous with an AC supply.
The present invention provides a solution to this and other problems which offers advantages over the prior art or which will at least provide the public with a useful choice.
All references, including any patents or patent applications cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the references states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinency of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents form part of the common general knowledge in the art, in New Zealand or in any other country.
It is acknowledged that the term ‘comprise’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprise’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or elements. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.
The term “motor phase” as used herein refers to the phase angle between the AC voltage applied to the motor and the back electro-motive force (EMF) produced by the motor. This value is normally closely equivalent to the phase relationship between rotor position and applied AC voltage for any one construction of a motor.